Jaagsiekte sheep retrovirus (JSRV) and the related small ruminant betaretroviruses are a fascinating group of viruses with unique characteristics that are of broad interest through their veterinary, comparative medical and biological importance. JSRV is the causative agent of ovine pulmonary adenocarcinoma (OPA), an important viral disease of sheep and an outbred animal model for human lung adenocarcinoma. The mechanisms followed by JSRV to transform cells are unique among oncogenic retroviruses as the viral envelope glycoprotein (Env) functions as a dominant oncoprotein. The enzootic nasal tumor virus (ENTV) is also an oncogenic retrovirus highly related both phylogenetically and biologically to JSRV. ENTV induces a contagious tumor of the mucosal nasal glands in sheep and goats. Interestingly, sheep, goats, and most species belonging to the subfamily of the Caprinae, have stably integrated in their genome endogenous retroviruses highly related to ENTV and JSRV (hence the name enJSRVs). enJSRVs are expressed, particularly in the genital tract epithelia of the ewe. Intriguingly, enJSRVs appear critical for sheep placental development during the pre-implantation period and therefore represent a powerful example on how viruses can shape mammalian biology.